A hollow fiber membrane is typically prepared by extruding a polymer solution for forming a fiber from an outer circular part of a double tube nozzle, concurrently with a core liquid including a lipophilic liquid, such as liquid paraffin, or a core gas, such as nitrogen gas, from the center of the nozzle to form a hollow fiber, while forming micro-pores in the membrane at the same time. Then, the hollow fiber membrane is washed with water, impregnated with an aqueous solution of glycerol in the fine pores therein, and wound up in a bobbin. Thereafter, a fiber bundle comprising a number of such hollow fibers is housed in a plastic case to give a hollow fiber membrane module.
It is a common practice to wash a hollow fiber membrane with ether, chlorofluorocarbon and the like to remove the core liquid in the hollow fiber or waste, dust and the like present in the core (hollow) part of the hollow fiber before or after forming the hollow fiber membrane module. However, ether may possibly start a fire, since it has a low boiling point and a low flash point. Moreover, washing with ether results in removal of glycerol filled in the fine pores in the hollow fiber membrane, thus lowering the water-permeability of the hollow fiber membrane after washing. In addition, ether easily produces peroxides upon heating and said peroxides exert adverse influence on the hollow fiber to cause degradation of the fiber.
A method comprising washing with a chlorofluorocarbon, such as trichlorotrifluoroethane having a high ozone depletion coefficient of 0.8, leads to the depletion of ozone layer, which in turn worsens the global environment, so that the use of trichlorotrifluoroethane is now restricted as being among specific fleons. While a chlorofluorocarbon such as dichloropentafluoropropane having a low ozone depletion coefficient of 0.2 may be used, this compound is associated with a drawback that it not only degrades the plastic case containing a hollow fiber bundle, but washes away glycerol filled in the fine pores in the hollow fiber membrane.
It is therefore an object of the present invention to overcome the problems of inflammability and pollution of global environment and to provide a method for washing a hollow fiber membrane to remove waste, dust and the like, which are present in the core part of the hollow fiber, and selectively remove a lipophilic liquid present in the core part of the hollow fiber without removing glycerol, polyethylene glycol and the like filled in the fine pores of the hollow fiber membrane, or degrading the case containing the hollow fiber bundle.